DIVERSITY  OF 


NQN  CIRCULATING 

CHECK  FOR  UNBOUND 
CIRCULATING  COPY 


UNIVERSITY  OF  ILLINOIS 


Agricultural  Experiment  Station 


BULLETIN  No.  265 


TULIP   BLOSSOM    BLIGHT 

BY  FRANK  LINCOLN  STEVENS  AND  O.  A.  PLUNKETT 


A  BLIGHTED  TULIP  SHOWING  WITHERING  OF  STALK 
AND  WOOLLY  GROWTH  OF  MYCELIUM 
ON  DISEASED  PART 


URBANA,  ILLINOIS,  APRIL,  1925 


FIG.  1. — HEALTHY  TULIP  (CENTER)  AND  Two  BLIGHTED  TULIPS  SHOWING 
BLOSSOM  INFECTION  AND  INFECTION  AT  LEAF  AXIL 


TULIP   BLOSSOM   BLIGHT 

By  FRANK  LINCOLN  STEVENS  and  0.  A.  PLUNKETT,  in  Cooperation  with  the 
DEPARTMENT  OF  HORTICULTURE1 

Tulip  blossom  blight  is  a  disease  which  manifests  itself  by  a 
blighting  and  wilting  of  the  flower  stalk  just  below  the  blossoms,  fol- 
lowed by  a  dropping  over  and  collapse  of  the  floral  parts  (cover  illus- 
tration). In  extremely  damp  weather  and  where  there  is  an  abundance 
of  shade,  the  petals  decay  rapidly,  becoming  covered  with  a  cottony 
mycelial  growth,  while  the  flower  stalk  undergoes  a  watery  .decay. 
The  disease  is  caused  by  a  fungus  heretofore  unreported  in  connection 
with  this  host. 

The  disease  was  first  brought  to  the  attention  of  the  plant  pathol- 
ogy laboratory  at  the  University  of  Illinois  in  May,  1924,  by  gardeners 
in  charge  of  the  University  grounds,  who  had  found  it  present  and 
destructive  in  a  number  of  tulip  beds.  A  survey  of  the  tulip  beds  on 
the  campus  and  in  the  surrounding  territory  has  revealed  the  disease 
in  only  six  beds  on  the  campus  and  as  prevalent  only  in  tulips  of  the 
double  varieties.  In  one  bed,  infection  to  the  extent  of  32  percent  of 
the  plants  was  found;  in  another  bed,  26  percent.  The  causal  fungus  has 
been  isolated  from  rotting  stems  and  leaves  of  single  varieties,  but  the 
typical  blossom  blight  has  not  been  observed  on  these  varieties  in 
nature. 

GENERAL  SYMPTOMS  OF  THE  DISEASE 

The  first  outward  manifestation  of  this  blossom  blight  of  tulips  is 
a  shriveling  of  the  flower  stalk  just  below  the  flower,  followed  by  a 
falling  over  of  the  blossom  (cover  illustration).  The  blossom  then 
withers  and  dries  up,  or,  if  conditions  are  favorable  for  the  disease, 
the  petals  become  covered  with  the  mycelium  of  the  fungus  and  a 
rapid  soft  decay  follows.  The  lesion  at  the  base  of  the  blossom  is  found 
to  be  shrunken  and  withered  and  of  a  yellowish-white  color  in  contrast 
to  the  green  of  the  normal  flower  stalk.  There  is  also  a  tendency  for  the 
stalk  or  blossoms  to  twist  at  the  lesions. 

A  second  and  less  common  method  of  attack  of  the  fungus  occurs 
lower  down  on  the  stalk,  near  the  base  of  the  leaf  (Fig.  1).  The 
symptoms  here  are  similar  to  those  described  above.  When  infection 


*Frank  Lincoln  Stevens,  Professor  of  Plant  Pathology,  University  of  Illinois;  O.  A. 
Plunkett,  Research  Assistant. 

299 


300 


BULLETIN  No.  265 


takes  place  in  the  leaf  axil,  the  leaves  sometimes  rot  off  at  the  base. 
If  the  tulips  are  growing  in  a  very  moist  situation,  and  conditions  are 
favorable  for  the  disease,  it  may  progress  downward  and  rot  off  the 
entire  stem.  In  most  cases,  however,  the  disease  occurs  as  a  lesion 
just  below  the  blossom  or  at  the  base  of  the  upper  leaves. 


FIG.  2. — TULIP  BLIGHTED  BY  INOCULATING  IN  BLOSSOM 

Study  of  a  number  of  plants  in  various  stages  of  infection  showed 
that  the  fungus  usually  gains  entrance  thru  the  blossom,  probably  at 
the  point  of  attachment  of  the  petals,  and  advances  into  the  stalk, 
causing  decay  and  withering.  In  cases  where  infection  occurs  lower  on 
the  stalk,  the  fungus  apparently  penetrates  thru  the  epidermis  in  the 
leaf  axil. 


7925]  TULIP  BLOSSOM  BLIGHT  301 

INFLUENCE  OF  ENVIRONMENT  ON  THE  DISEASE 

From  the  observations  made  by  the  authors  it  appears  that  the 
prevalence  of  this  disease  is  almost  entirely  dependent  upon  environ- 
mental conditions.  The  beds  in  which  the  disease  occurred  in  abund- 
ance were  all  in  damp,  shaded  situations.  Beds  in  open  and  well-lighted 
and  ventilated  areas  showed  no  evidence,  or  only  slight  evidence,  of  the 
disease.  The  mode  of  infection  bears  out  the  assumption  that  moisture 
is  a  necessary  factor  for  prevalence  of  the  disease.  The  double-blossom 
varieties  hold  moisture  for  forty-eight  hours  or  more  under  usual  con- 
ditions, while  the  single  varieties  dry  out  quickly.  This,  then,  appears 
to  be  the  reason  that  the  double  varieties  are  susceptible,  while  the 
single  varieties,  even  in  the  same  bed,  are  not.  The  unusual  amount  of 
rain  during  May,  1924,  gave  ideal  conditions  for  the  development  of  the 
disease. 

THE  CAUSAL  ORGANISM 

A  practically  pure  culture  of  fungus  was  noted  in  the  material  first 
examined  and  in  all  newly  collected  material,  and  was  found  constantly 
associated  with  the  typical  disease.  A  large  number  of  isolations  con- 
sistently gave  this  fungus,  either  in  pure  culture  or  in  great  preponder- 
ance over  any  other  growth.  The  isolations  were  readily  made  by 
planting  bits  of  diseased  tissue  directly  on  poured  corn-meal  agar  plates. 

PROOF  OF  PATHOGENICITY 

Inoculation  No.  1. — Recently  opened,  double  tulip  blossoms,  grow- 
ing outside  under  normal  conditions,  were  inoculated  by  placing  at  the 
base  of  the  petals  and  also  on  top  of  the  pistil  pieces  of  corn-meal  agar 
containing  the  fungus.  At  the  end  of  six  days  the  normal  type  of  blos- 
som blight  resulted  (Fig.  2). 

Inoculation  No.  2. — Tulips  of  a  single  variety  were  inoculated  in 
a  similar  manner  without  positive  results. 

Inoculation  No.  3. — Double  and  single  tulips  were  inoculated 
under  bell  jars  in  a  saturated  atmosphere.  All  of  these  showed  typical 
blossom  blight  at  the  end  of  three  days. 

Inoculation  No.  4. — Blossoms  of  Tradescantia  spp.  were  inoculated 
in  an  inoculation  case,  in  a  saturated  atmosphere.  None  of  the  blossoms 
of  this  host  became  infected. 

Inoculation  No.  5. — Green  tomato  fruits  were  inoculated  with  the 
fungus  by  puncturing  the  epidermis.  Ten  days  later  there  was  a  small 
rotten  spot  around  the  point  of  inoculation,  but  the  remainder  of  the 
tissue  remained  firm. 

Inoculation  No.  6. — Growing  stems  of  beans  and  growing  radishes 
were  inoculated  in  a  moist  chamber.  Seven  days  later  no  signs  of  infec- 
tion were  visible. 


302 


BULLETIN  No.  265 


[April, 


Inoculation  No.  7. — Iris  blossoms  were  inoculated  by  placing 
among  the  petals  chunks  of  corn-meal  agar  containing  the  fungus.  The 
blossoms  were  placed  in  an  inoculating  chamber  and  kept  in  a  saturated 
atmosphere  for  forty-eight  hours.  Five  days  later  the  blossoms  had 
fallen  over  and  the  peduncles  showed  lesions  similar  to  those  on  tulips. 


FIG.  3. — BLIGHTED  AND  HEALTHY  IRIS  BLOSSOMS;  BLIGHT 
CAUSED  BY  INOCULATION  WITH  A  PURE  CULTURE 


The  petals  were  soft  and  rotten.  Uninoculated  checks  remained  upright 
and  healthy  (Fig.  3). 

Inoculation  No.  8. — Growing  gladioli  bulbs  were  inoculated  by 
puncturing  the  epidermis  and  by  placing  agar  bearing  the  fungus  on 
uninjured  surfaces  and  on  roots.  The  plants  were  kept  under  bell  jars 
for  forty-eight  hours.  There  were  no  signs  of  infection  either  within  the 
bulbs  or  on  the  roots. 


19251 


TULIP  BLOSSOM  BLIGHT 


303 


Inoculation  No.  9. — Seeds  of  cotton,  flax,  sugar  beets,  and  onions 
were  planted  in  six-inch  pots.  Corn-meal  agar  containing  the  fungus 
was  mixed  well  with  soil,  and  a  covering  one  inch  in  depth  was  placed 
over  the  seeds.  The  soil  was  kept  well  moistened  under  bell  jars. 
Damping-off  occurred  in  pots  containing  sugar  beets  and  flax.  Check 
pots  remained  healthy. 

In  all  cases  where  positive  results  were  obtained  from  the  inocula- 
tions, reisolations  have  been  made  and  in  all  cases  the  fungus  isolated 
proved  to  be  identical  with  the  fungus  used  in  the  inoculum. 


DESCRIPTION  OF  THE  FUNGUS 

MYCELIUM 

The  mycelium  is  of  the  true  Phycomycete  type,  being  a  long  tube, 
irregularly  branched,  with  few  or  no  cross  walls.  Within  the  tulip 
tissue  and  in  artificial  media,  the  mycelium  is,  for  the  most  part,  thick 
and  profusely  branched,  measur- 
ing from  2.5  to  11  /x  in  diameter. 
The  hyphae  are  granular  and 
slightly  yellowish  in  color.  In  cul- 
ture and  on  the  normal  host,  the 
aerial  mycelium  is  much  finer  and 
less  branched  than  that  within  the 
host  tissue  or  within  artificial 
media.  In  culture,  the  mycelium 
sometimes  becomes  prominently 
warty,  owing  to  the  development 
of  numerous  small  protuberances 
(Fig.  4). 

CONIDIA 

Conidia  are  abundant  in-  cul- 
tures on  oat-meal  agar  and  are 
usually  borne  above  the  sub- 
stratum, apically,  on  thin  hyphae 
which  branch  off  from  the  larger 
mycelium  with  the  agar.  The 
conidia  are  slightly  variable  in 
size  and  shape,  but  are  compara- 
tively large  and  short-ovate  in 
form.  They  measure  from  22  to 
32  by  21  to  29  /«,.  Distinct  papil- 
lae are  always  present  on  these  conidia  and  are  mostly  apical  (Fig.  5), 
tho  occasionally  a  lateral  one  may  be  observed.  They  are  about  3.6  /x  in 
height  by  5.4  /x  wide.  In  rare  instances  two  papillae  occurred  on  one 


FIG.  4. — MYCELIUM  FROM  WITHIN   THE 

MEDIUM,    SHOWING    COARSE   AND 

KNOTTED  APPEARANCE 


304 


BULLETIN  No.  265 


[April, 


conidium.  No  intercalary  conidia  have  been  ob- 
served, and  the  conidia  are  never  borne  on  the 
heavy  mycelium  but  on  thin  branches.  When 
placed  in  sterile,  distilled  water  these  conidia 
germinate  by  an  ordinary  germ  tube  which  gives 
rise  to  the  normal  mycelium  (Fig.  6).  Repeated 
attempts  to  obtain  zoospores  have  failed. 

Oospores  are  abundant  in  pure  culture  and 
within  the  host  tissues.  They  are  spherical,  rang- 
ing from  22  to  32  /A  in  diameter.  The  oospores  are 
thick  walled  (1  to  2  /A),  mostly  hyaline,  tho  some- 
times distinctly  yellowish  brown,  depending  upon 
the  substratum.  They  are  borne  within  oogonia,  on 

short  side  branches  of  the  thick,  heavy  mycelium  inside  the  medium  or 
host  tissue.  Antheridia  are  always  present  and  are  persistent  and  pre- 
ponderantly paragynous.  They  may  be  borne  on  the  same  or  on  differ- 
ent mycelium  from  that  of  the  oogonia.  The  antheridia  are  variable  in 
shape,  ranging  from  long  curved  tubes  to  irregular  and  spherical  bodies 
(Fig.  7). 

Chlamydospores   have  not  been  seen  either  in  culture  or  in  the 
normal  host  tissues. 


FIG.     5. — A    SINGLE 
CONIDIUM,  SHOW- 
ING  PAPILLA 


FIG.  6. — GERMINATION  OF  THE  CONIDIA 


1925} 


TULIP  BLOSSOM  BLIGHT 
OOGONIA 


305 


Oogonia  are  borne  on  short  side  branches  on  the  large  mycelium 
within  or  on  the  surface  of  the  medium.  In  nature  the  oogonia  are  borne 
within  the  host  tissue. 


FIG.  7. — DRAWINGS  SHOWING  OOGONIA,  OOSPORES,  AND  ANTHERIDIA 


CULTURAL  CHARACTERS 

Growth  on  Corn-Meal  Agar. — On  corn-meal  agar  there  is  a  trans- 
lucent growth  of  the  fungus,  giving  a  granular  appearance  to  agar. 
Here  and  there  over  the  surface,  flakes  of  white  appear  where  aerial 
hyphae  are  sent  up  bearing  conidia.  The  fungus  penetrates  up  to  4  mm. 
in  this  agar  and  produces  abundant  oogonia  at  this  depth.  The 
mycelium  shows  more  irregularity  on  this  medium  than  on  any  other. 
Oogonia  and  conidia  are  not  so  abundant  as  on  oat-meal  agar.  There  is 
no  aerial  mycelial  growth  excepting  the  very  short  conidiophores  which 
are  sent  up  from  within  the  medium.  The  conidiophores  are  much 
shorter  on  this  medium  than  on  oat-meal  agar.  Growth  is  very  rapid,  a 
colony  occupying  a  plate  within  four  to  six  days  after  inoculation. 

Growth  on  Tulip-Stern  Agar. — On  tulip-stem  agar  the  growth  is 
similar  to  that  on  corn-meal  agar,  but  not  so  abundant.  There  is  no 
aerial  mycelium  and  the  mycelial  development  is  entirely  within  the 
medium.  The  rate  of  growth  is  rapid,  similar  to  that  on  corn-meal  agar. 
No  conidia  are  produced*  on  this  medium  and  the  development  of 
oogonia  is  not  so  abundant  as  on  corn-meal  and  oat-meal  agars. 

Growth  on  Oat-Meal  Agar. — On  oat-meal  agar  the  growth  of  the 
fungus,«  tho  slower  in  spreading  over  an  agar  plate,  is  much  thicker  and 
the  mycelium  more  abundant  than  on-  corn-meal  agar.  A  thick  weft  of 
white,  aerial  mycelium  is  present.  It  is  on  this  thin,  white,  aerial 
mycelium  that  conidia  are  produced  in  abundance.  The  aerial  mycelium 
is  produced  in  quantity  only  when  the  cultures  are  several  days  old  and 


306  BULLETIN  No.  265  [April, 

are  grown  in  the  light.  When  grown  in  darkness,  only  a  slight  aerial 
mycelium  is  produced.  Small,  white,  irregular  spots  that  are  granular 
are  visible  within  the  agar.  .These  spots  are  masses  of  oogonia  which 
are  produced  in  more  abundance  on  this  agar  than  on  corn-meal  or 
tulip-stem  agar. 

Growth  on  Sterilized  Vegetable  Plugs. — An  abundant  growth  of  the 
fungus  is  produced  on  sterilized  potato  plugs,  the  mycelium  forming  a 
weft  completely  covering  the  plug.  The  growth  is  white  and  aerial 
mycelium  is  very  abundant.  Scattered  conidia  may  be  found  within  the 
aerial  mycelium.  Oogonia  are  produced  in  abundance  on  the  mat  of 
mycelium  lying  on  and  within  the  outer  surface  of  the  plug.  Oogonial 
development  is  not  so  rapid  as  on  carrot  or  bean  plugs. 

Growth  of  the  fungus  is  more  rapid  on  carrot  plugs  than  on  potato, 
but  no  aerial  mycelium  is  produced.  The  surface  of  the  plugs  appears 
gray  and  powdered,  as  tho  sprinkled  over  with  meal.  This  powdered 
appearance  is  caused  by  the  abundant  clumps  of  oogonia.  The  mycelium 
forms  a  mat  around  the  plug,  penetrating  the  outer  cells  for  a  milli- 
meter or  so. 

On  green  bean  plugs  the  fungus  grows  rapidly,  but  only  a  small 
amount  of  mycelium  is  formed.  Oogonia  are  produced  earlier  on  these 
plugs  than  on  potato  or  carrot  but  are  not  so  abundant.  Slight  aerial 
mycelium  is  present  at  first,  but  as  growth  continues  the  bean  tissue 
becomes  watery  and  rots.  Conidia  have  not  been  observed  on  this 
medium. 

TAXONOMIC  POSITION  OF  THE  PARASITE 

The  morphology  of  the  oogonia,  oospores,  and  conidia  clearly 
places  this  fungus  as  a  Phytophthora.  In  this  genus  the  preponderance 
of  paragynous  antheridia,  the  oospore  measurements,  and  the  pre- 
ponderance of  oospores  in  the  early  growth  of  the  cultures  show  that  it 
is  closely  allied  to,  if  not  identical  with,  P.  cactorum.  The  characters 
given  above  make  it  impossible  to  regard  the  fungus  as  cospecific  with 
any  other  known  species.  In  view  of  its  close  similarity  with  P.  cactorum 
it  has  been  referred  to  that  species. 


.7925]  TULIP  BLOSSOM  BLIGHT  307 


SUMMARY 

1.  An  hitherto  unreported  tulip  disease  is  described. 

2.  The  causal  fungus  is  a  Phytophthora  which  is  referred  to  the 
species  P.  cactorum. 

3.  Its  etiological  relation  to  the  disease  was  indicated  by   (a)   its 
constant  presence  in  the  diseased  tissue;  (b)  the  absence  of  any  other 
constant    parasite;     (c)     proof    of    its    pathogenicity    by    inoculation; 
(d)  reisolation  of  the  organism. 

4.  The  disease  is  more  common  in  humid,  ill-ventilated  situations. 

5.  It  is  more  common  on  double  than  on  single  blossoms,  appar- 
ently because  of  humidity  differences. 

6.  Iris  was  susceptible  to  inoculation. 


RECOMMENDATION 

The  only  suggestion  which  can  be  made  as  to  the  prevention  of 
the  fungus  is  the  avoidance  of  situations  that  are  especially  humid. 


UNIVERSITY  OF  ILLINOIS-URBANA 


